The present invention is directed to novel polycarbodiimide compounds, their method of preparation, and their use in one-package, ambient cure automotive paint compositions. It is often desirable to be able to apply several ambient curable coating technologies over the substrates typically found on an automobile. These substrates could range from bare metal to plastics to the original equipment manufacturer's (OEM) coatings. The ambient curable coating technologies may range from thermoplastic acrylic lacquers to air-oxidizing alkyds to two-component acrylic urethanes.
In order to achieve this range of substrate adhesion, an intermediate coating, also known as a "mid-coat" or "tie-coat", is applied to the substrate prior to topcoating. This intermediate layer needs to be adherent to the substrate well as the top coating to be effective. The mid-coat must also develop these adherent properties rapidly, in order to prevent a tearing of the applied layer once the masking tape is removed in multi-layer or multi-"tone" applications, and retain its adherent qualities even after being subjected to the elements of exterior exposure, such as high and low temperatures, high humidity conditions and ultraviolet radiation. An ideal mid-coat would also ensure proper adhesion on substrates that are improperly prepared.
In addition to the aforementioned properties, there are many desirable attributes of a mid-coat adhesion promoter. Aside from being adherent, the coating needs to be transparent and color stable, as to not affect the color of the substrate in cases where only a clear coating is applied, one-package, for ease of use, and retain its film integrity as a topcoat, because in many applications, the adhesion promoter is applied beyond the repair area.
Carbodiimide compounds, particularly aliphatic carbodiimides, are known to be reactive with a number of functional groups such as carboxyl, amine, mercaptan, epoxy, isocyanate and hydroxyl, groups that appear frequently on substrate surfaces and in coating technologies. This property, in addition to the excellent wetting behavior of carbodiimides prepared from typical isocyanates make them well suited as primer vehicles. Carbodiimide polymers are well known in the art as being well suited as a hardening component for two-package primer vehicles. U.S. Pat. No. 4,118,536 describes the use of a one-package carbodiimide primer prepared exclusively from aromatic isocyanates and terminated with alkoxysilanes. Polycarbodiimides prepared from aromatic isocyanates are generally high in color and of low reactivity. Furthermore, undesirable UV induced effects are common when aromatic groups are conjugated with an isocyanate or the reaction products of isocyanates, due presumably to the creation of quinoid-like structures. Mixed aliphatic/aromatic polycarbodiimides may also be high in color, depending on the relative amount of the aromatic component, require special processing and are suspect for UV durability for reasons just mentioned. The invention described herein has rapid adhesion to a wide variety of substrates, does not necessarily require alkoxy silane to be effective, is a one package application, transparent, weathers well and provides excellent humidity and ultimate adhesion over a wide variety of scuffed and unscuffed substrates.